Method and apparatus for counting particles in a gas

ABSTRACT

The present disclosure describes a method and apparatus for detecting particles in a gas by saturating the gas with vapor and causing the gas to flow through a chamber with walls that are at a temperature different than the temperature of the entering gas creating a gas turbulence within the chamber resulting in the gas becoming super-saturated with vapor and causing said super-saturated vapor to condense on said particles and form droplets, which are then detected and counted by an optical light-scattering detector.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 12/872,697, filed Aug. 31, 2010 which is based onand claims the benefit of U.S. provisional patent application Ser. No.61/252,243, filed Oct. 16, 2009, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND OF THE DISCLOSURE

The present disclosure describes a method and an apparatus for detectingparticles in a gas by condensing vapor on the particles to formdroplets, which are then detected by an optical, light-scatteringdetector. Instruments using vapor condensation on particles to formdroplets for detection are referred to as condensation particle counters(CPC), or as condensation nucleus counters (CNC). A variety of workingfluids can be used to generate vapor for condensation. The most commonworking fluids are butyl alcohol and water.

Condensation particle counters are useful in many applications. In airpollution and climate research, for instance, the instrument is oftenused with an electrical mobility analyzer to determine the concentrationand size distribution of particles in the ambient atmosphere. Theinstrument can also be used to detect particulate contaminants suspendedin clean-room air for clean-room monitoring and contamination controlpurposes. In addition, CPC is widely used in laboratory research tostudy the property and behavior of small airborne particles.

The most important process in a CPC is the process of vapor generation,condensation and droplet growth. The present disclosure describes a newapproach to creating super-saturation for vapor condensation and dropletgrowth, leading to a compact measuring device with improved performancecharacteristics.

SUMMARY OF THE DISCLOSURE

The present disclosure describes a method and apparatus for detectingparticles in a gas by saturating the gas with vapor at one temperatureand causing the gas to flow through a chamber with walls at a differenttemperature, thereby changing the gas temperature in the chamber. At thesame time the gas flow is made turbulent causing the gas to mix in thechamber to create super-saturation for vapor to condense on saidparticles and form droplets, which are then detected and counted by anoptical light-scattering detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the system for detecting particles in agas by vapor condensation on particles to form droplets and detectingthe droplets so formed by an optical light-scattering detector.

FIG. 2 is a longitudinal sectional view of saturator 300 of FIG. 1.

FIG. 3 is a sectional view of saturator 300 of FIG. 1 in a transversedirection to the direction of gas flow.

FIG. 4 is a vertical sectional view of condenser 400 of FIG. 1.

FIG. 5 is a sectional view of in another embodiment of a condenser.

FIG. 6 is a sectional view of condenser 400 showing circulatory gas flowcreated by a gas jet impinging on the wall of a cylindrically shapedchamber in a perpendicular direction.

FIG. 7 is the same sectional view of condenser 400 as shown in FIG. 6but illustrating a gas jet with small linear momentum creating acirculatory gas flow along the cold condensing wall ending with the flowbecoming mixed prior to reaching the end of the full flow path along thecold condenser wall.

FIG. 8 is the same sectional view of condenser 400 as shown in FIG. 6with a tangential gas flow inlet to create a circulatory gas flow in thechamber causing the gas to cool and mix leading to vaporsuper-saturation, condensation and droplet growth

FIG. 9 is a vertical sectional view of a conical condenser.

FIG. 10 is a graphical view illustrating the basis of the theoreticalanalysis leading to the results shown in FIG. 11.

FIG. 11 is a graphical view illustrating the theoretical saturationratio, S created in the circulatory gas flow condenser of the presentdisclosure showing the dependence of S on the cooling parameter, f.

FIG. 12 is a graphical view illustrating the theoretical relationshipbetween the saturation ratio, S, and the minimum diameter, d, ofparticles on which vapor will condense to form droplets for detectionaccording to the Kelvin equation, with the diameter, d, being commonlyreferred to as the Kelvin equivalent diameter.

FIG. 13 is a sectional view of another embodiment of a condenser

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a condensation particle detecting apparatus generallyindicated at 90 according to the present disclosure. The apparatus 90includes a pump, 100, to maintain a steady gas flow through the systemand allow gas carrying suspended particles from source 200 to flowthrough, the pump speed being adjustable in order to adjust or controlthe rate of gas flow to a desired value. Source 200 can be the ambientatmosphere, the atmosphere of a clean-room, or simply a source ofgas-borne particles being sampled into the system for detection. Theapparatus 90 also includes a saturator, 300, to saturate the gas withvapor and a condenser, 400, to create a super-saturated vapor atmospherein which vapor can condense on particles to form droplets for detectionby the downstream detector, 600.

In addition, the apparatus 90 includes an electronic controller, 700,with the needed circuitry to receive output signal from various sensorsin the system 90l and provide the needed control to operate theindividual system components in a consistent and repeatable manner.Typical parameters that need to be monitored and controlled includetemperature, flow rate, liquid level, and those pertaining to dropletdetection by light scattering, such as the output power of the laserlight source, sensitivity of the photo-diode or photomultiplierdetector, gain and threshold settings for the detecting circuitry, amongothers. All system components such as pump 100, saturator 300, condenser400, and detector 600 are provided with the requisite input and outputlines 101, 301, 401, and 601 to communicate with controller 700 andmaintain the individual system components to the desired operating pointin the sensed and controlled parameters, controller 700 being providedwith its own input and output lines 701 and 702 for communication andcontrol purposes.

The apparatus of this disclosure is designed so that gas-borne particleswill form the nucleus of condensation in a super-saturated vaporatmosphere to condense vapor on particles and form droplets fordetection. The particles forming the nucleus of condensation are toosmall to be detected directly by light scattering detector 600. Theparticles are detected indirectly as a result of vapor having condensedon the particles to form droplets of a considerably larger size, therebymaking the small non-detectable particles detectable by lightscattering. In the ambient atmosphere, for instance, there are numerousparticles smaller than about 100 nm in diameter. Such particles aregenerally very difficult to detect by an optical, light scatteringapproach. By growing the particles to a larger droplet size by vaporcondensation and droplet growth and detecting the grown droplets bylight scattering, the existence of the small condensation nuclei in thegas can be confirmed. The apparatus 90 is therefore also known by thename “condensation nucleus counter,” or CNC. The saturator and condenserin a condensation particle counter can thus be viewed as a particle sizemagnifier helping to make a small, non-detectable particle into a largedetectable droplet by vapor condensation and droplet growth.

The CPC is a unique instrument and an important device for aerosolmeasurement. In scientific usage, aerosol refers to a gas containingsuspended particles. The most common gas is air. Other gases, such asnitrogen, helium, hydrogen, oxygen, etc. can also be the gaseous mediumin which the particles are suspended. The CPC, therefore, is useful notonly for airborne particle measurement, but for the measurement ofgas-borne particles in general.

FIG. 2 is a longitudinal sectional view of saturator 300 along thedirection of gas flow as indicated by arrows 302. A sectional view inthe transverse direction perpendicular to the direction of gas flow isshown in FIG. 3. Saturator 300 is provided with an enclosure 305 withattached electric heater 320 and temperature sensor 330 in order tocontrol the enclosure temperature to a desired set-point value.Enclosure 305 forms an envelope around chamber 310 in which a liquid isplaced to generate vapor for saturating a gas flowing through thechamber. FIG. 3 shows a liquid working fluid 340 which is filled tolevel 345 in the chamber. In addition to the working fluid 340, whichprovides the source of vapor, a multitude of porous plates 350,preferably porous metal plates, is placed in the chamber and partiallyimmersed in the liquid. The porous plates are positioned substantiallyparallel to the flow of gas through the chamber. By virtue of capillarysurface tension, the interstitial pore space of the porous panels isfilled with working fluid 340 including the portion of the porous panelin the gas-filled space above liquid surface 345. As a result, theporous panels are filled with liquid working fluid 340 thereby makingthe porous panels fully wetted by liquid. Since all parts of the systemare in close thermal contact with one another, the entire assembly,including the enclosure 310, porous metal panels 350, and working fluid340 contained therein are heated to substantially the same uniformtemperature.

Enclosure 305 is provided with an inlet, 360, for the gas carryingsuspended particles to enter and an outlet, 365, for the gas to exit. Asgas carrying suspended particles flows through the gas flow passagewaysbetween the liquid-filled, wet porous panels 350, the gas is heated bythe hot, wet porous panels 350. At the same time, liquid evaporatingfrom the wet panel surface will generate vapor to saturate the gas (withsuspended particles) with vapor. The gas, upon exiting the saturatorthus becomes heated and is substantially saturated with vapor. Thesaturator is designed so that the gas will attain a selected temperatureT₁ and is substantially saturated with vapor at that temperature uponits exit from the saturator. This vapor saturated gas with the suspendedparticles then flows into chamber 400 to create super-saturation forvapor condensation and droplet growth. Saturator 300 can also bedesigned with a pre-heater to pre-heat the gas to substantially the sametemperature as the saturator. The gas so preheated can then be saturatedwith vapor as it flows along the liquid-filled, wet porous panels in thesaturator. Either of the above approaches can be used to heat the gasand saturate the gas with vapor.

The design of saturator 300 in FIG. 2 and FIG. 3 shows heater 320 to beattached to the top of enclosure 310 to insure that the top of theenclosure is slightly warmer than the remaining parts of the saturator.Vapor evaporating from the liquid surface 345 and from the surface ofthe porous panels, 350, in the enclosure will thus encounter a slightlywarmer inside surface 315 at the top and not condense there. Vaporcondensing on the inside top surface 315 would form accumulated liquidwhich would fall back to liquid 340 below to cause splashing, whichwould generate small droplets that can be carried by the gas flow intothe condenser to create a false particle count. For particle detectionin a clean-room with low airborne particle concentration, a false countwill contribute to the measurement error and must be avoided.

A multitude of porous panels 350 is provided in saturator 300 in orderto provide a multitude of wet panel surfaces to generate vapor. By usinga large number of closely spaced panels substantially parallel to theflow of gas as shown in FIG. 3, a very large wetted surface area can becreated in a small physical volume, thus allowing a high volume of gasto flow through and be saturated with vapor. At low volumetric rate ofgas flow, a single porous panel is sufficient. A single panel immersedin liquid will provide two wetted surfaces above the liquid from whichthe liquid can evaporate to generate vapor, which then diffuses into thegas stream flowing nearby. At higher gas flows, a multitude of platesare used to provide enough surface area for vaporization. In oneimplementation of the saturator, ten plates spaced 3 mm apart from eachother have proven sufficient to achieve the desired vapor saturationresults needed for the application. The parallel panels will thus makeit possible to design a CPC 90 with a high volumetric flow rate of gasin order to detect particles having a low airborne concentration such asthat in a clean-room.

Traditional CPCs are designed with the liquid evaporating from awet-wall saturator with the gas flow passageway being surrounded by aporous material wall filled with the working fluid. The flow passagewaycan have a rectangular cross-section, as shown in U.S. Patent No.4,790,650, or a tubular flow passageway with a circular cross-section asshown in U.S. Pat. No. 6,829,044B2. In these traditional saturatordesigns, liquid can only evaporate from the wet rectangular or circularwalls surrounding the gas-flow passageway with no additional surfacesbeing provided to generate vapor for saturating the gas.

In the vertical parallel panel design of the present disclosureadditional evaporative surface areas are provided within the overallenvelope of the gas-flow passageway walls to generate vapor forsaturating the gas. This approach greatly increases the surface areathat can be placed in the gas flow passageways to saturate a high rateof gas flow through the passageways. The design of this disclosure isalso very flexible making it possible to increase the wet evaporativesurface to as large an area as necessary in order to saturate the gas atany desired rate of gas flow. The lack of a suitable saturator with anadequate evaporative capacity has limited the maximum gas flow rate thatcan be achieved in the traditional CPC design in the past. Suchflexibility is now provided in the saturating apparatus design of thepresent disclosure.

FIG. 4 shows the vertical sectional view of condenser 400 in oneembodiment. The condenser is made from a rectangular shaped metal piececontaining a cylindrically shaped chamber 410 with the chamber's axis415 placed in a vertical orientation. Chamber 410 has an inlet 420 for agas to enter and an outlet 425 for the gas to exit. Inlet 420 is locatedon the vertical side wall of the cylinder, while outlet 425 is locatedon a top wall. A thermoelectric cooling (TEC) module 430 is placed inclose thermal contact with metal piece 405 to cool the metal piece to adesired operating temperature, T₂. Cooling is achieved by applying a DCvoltage to cause a DC current of a specified polarity to flow throughTEC module 430 to create the desired thermoelectric cooling effect. Heatgenerated by the thermoelectric cooling module is dissipated to theambient by an extended surface heat exchanger, 435, by natural or forcedconvection. As a result, the metal piece and the metal wall 440 ofchamber 410 are also cooled to substantially the same temperature T₂. Atemperature sensor, 445, in contact with metal piece 405 senses thetemperature of the metal piece. With the aid of controller 700, theelectric power input to TEC module 430 can be varied to allowtemperature T₂ to be controlled to a desired set-point value.

In some cases, the condenser 400 may need to be heated to a temperatureabove the surrounding environment in which the condenser is placed. Inwhich case, the current flow can be reversed by applying a voltagehaving a polarity opposite to that needed for cooling. TEC module 430,therefore, can be placed in the heating mode working as a heat pump topump heat from the surrounding environment to heat the condenser.Alternatively, a separate electric heater can be used to provide heatingto the condenser, and keeping it at a temperature above the surroundingenvironment.

As the gas containing vapor and suspended particles flows through inlet420 into the thermoelectrically cooled condensing chamber 410, therelatively warmer gas encounters the relatively cooler condenser chamberwall 440, thereby causing the relatively warmer gas to cool forming agas stream having a non-uniform temperature distribution. Thetemperature distribution in the gas becomes non-uniform because the gasstream moving closest to the cold condenser wall 400 will lose more heatthan gas streams that are farther away from the wall. At the same time,vapor in the gas will diffuse to the chamber wall 440 to condense on thewall. The vapor concentration in the gas will also become non-uniformsince the gas stream moving closest to the wall would lose more vapor bycondensation on the wall than streams that are farther away. As aresult, the vapor concentration in the gas will also become non-uniform.This non-uniformly cooled gas stream having a non-uniform temperatureand a non-uniform vapor concentration profile then mixes when the gasflow becomes turbulent thereby creating a mixture having a more uniformtemperature and vapor concentration. The result is the formation of asuper-saturated vapor atmosphere in which vapor can condense onparticles to form droplets. The mixture then flows out of chamber 410through outlet 425 to droplet detector 600 located downstream ofcondenser 400 as shown in FIG. 1.

The approach to creating vapor saturation by cooling and mixing a gas ina cold-wall chamber in the manner described above is previously unknown.The theoretical basis of such an approach will be explained more fullylater in this disclosure.

An embodiment with additional features of the condenser 400 is shown inFIG. 5. The length of cylindrical chamber 410 is extended by providing asecond condensing chamber 426 downstream of condensing chamber 410.Outlet 425 for chamber 410 then becomes the inlet for chamber 426.Outlet 425 is smaller in cross-sectional area than chambers 410 and 426,thus forming a restrictive flow passageway for the gaseous mixture fromchamber 410 to flow through and enter chamber 426. The gas flow in thisrestrictive flow passageway is sufficient to create a turbulent gas jetdownstream of the restriction. The fluid turbulence in the gas jetcreates additional mixing to make the gas mixture in chamber 426 morehomogeneously mixed than the mixture in chamber 420. In addition,chamber 426 provides additional volume for the gas to reside and flowthrough, thereby increasing the overall residence time of the mixture inchambers 410 and 426 to give the droplets more time to grow to a largersize prior their exit through outlet 428 with the gas flow to thedownstream droplet detector 600.

FIG. 6 is a horizontal sectional view of FIG. 4 showing thecylindrically shaped chamber 410 along section A-A of FIGS. 4 and 5.Thermoelectric module 430 and heat sink 435 are similarly labeled inboth FIGS. 4 and 5 and FIG. 6. As the warm vapor laden gas withsuspended particles enters through inlet 420, the gas with suspendedparticles forms a gas jet travelling along path 425 through theintervening cylindrical chamber space to impinge on the opposite wall ofthe chamber. Upon hitting wall 440 the gas jet is deflected side ways tocreate a circulatory gas flow along paths 445 and 450. As the gas flowsalong these paths, it loses heat to the adjacent cold condenser wall 440by convection. At the same time, the vapor will also condense on thesurface of the cold condenser wall. The condensed liquid will drain bygravity along the condenser wall to a liquid reservoir below, which isnot shown. The condenser 400 is maintained at a relatively coolertemperature, T₂, compared to the temperature T₁ of saturator 300. At theend of the circulatory gas flow pathways, or streamlines, 445 and 450,the gas, having cooled by contact with wall 440, and having lost somevapor by condensation, will have a non-uniform temperature distributionacross the streamlines. As used herein, flow paths and streamlines aresynonymous terms in laminar flow, the fluid does not inter-mix. Thefluid travels in a well defined path to form a well defined streamline.In turbulent flow, there is turbulent mixing of the fluid. In whichcase, the flow paths, or streamlines refer to the pathway or streamlineof the mean flow. Superimposed on the mean flow pathway or streamlineare turbulent eddies that cause the flow to move rapidly in the lateraldirection. At the same time the partial pressure of vapor in the gasstream will also become non-uniformly distributed across the streamlinesdue to the varying amount of vapor that has diffused across thestreamlines to condense on the wall of the cold-wall condenser. Uponreaching the end of flow paths 445 and 450, the remaining kinetic energyof the flowing gas stream will cause the gas stream to breakup intoturbulent eddies, filling the space in the core of the circulatory gasflow with a turbulent mixture. The turbulent gas flow is in theturbulent core. The turbulent gas flow helps make the mixture becomemore uniformly mixed with a substantially uniform temperature andpartial vapor pressure in the gas. The flow in this circulatory gas flowcondenser is therefore in the form of a vortex flow with a relativelyhigher velocity gas flow circulating around a turbulent vortex core toproduce the mixing needed for the vapor to become super-saturated andcondense on particles to form droplets.

The gas flow pattern depicted in FIG. 6 is that from a gas flowing at arelatively high volumetric rate of flow through an inlet 420 with arelatively small cross-sectional area, thereby creating a high velocitygas jet with a high linear momentum in the direction of the gas jet.This high momentum gas jet upon traveling to the end of the circulatoryflow paths 445 and 450 adjacent to the condenser wall would have lostsome linear momentum due to fluid friction, but would still retainsufficient momentum for the gas to flow in the forward direction,causing it to spiral inward as shown by arrows, 452, 454, and 456,toward the center 460 of a vortex as depicted in the top half of FIG. 6.As the gas flow spirals inward toward the center 460, it will continueto shed turbulent eddies to dissipate the kinetic energy carried by theflow, thereby creating a turbulent region in the vortex core. The vortexflow depicted in the top half of FIG. 6 has a counter clockwiserotation. The vortex formed in the bottom half the FIG. 6 is similar tothat in the upper half except the direction of rotation of the vortex isclockwise.

In contrast to the above, a gas entering the chamber at a relatively lowvolumetric rate of gas flow through an inlet with a relatively largercross sectional area will carry less momentum. Such a gas would form ajet travelling through the cylindrical chamber as in FIG. 7 at arelatively low speed. The gas jet, upon hitting wall 440 would bedeflected sideways to flow along paths 445 and 450 adjacent to the coldchamber wall 440. The momentum of the flowing gas stream is too low forit to flow beyond the end of the path at 412 and 414. Upon reaching theend of flow paths 412 and 414, the gas, having lost much of its forwardmomentum will flow to the vortex core region with much less turbulentmixing compared to that depicted in FIG. 6. Mixing will still occur, butwith a lesser intensity than that depicted in FIG. 6.

To prevent the gas flow in chamber 410 in the circulatory gas flowcondenser of FIG. 4 from flowing out of chamber 410 prior to sufficientmixing has taken place, the top of the chamber is provided with outlet425 with a smaller cross-sectional area than that of the cylindricalchamber to confine the circulating gas flow in chamber 410 and cause itto mix prior to its exit from the chamber, thus helping the gas flowingout of the chamber to be uniformly mixed. As discussed earlier, a secondchamber can also be provided as shown in FIG. 5 to provide additionalvolume for the mixture to undergo additional mixing and provide moreresidence time for the droplets to grow in the chamber.

The jet of gas issuing out of orifices 420 and 425 will be turbulent ifthe Reynolds number of the flow is larger than about 100. In comparison,the gas flow in a circular tube typically does not become fullyturbulent until the flow Reynolds number exceeds 2300. The flow Reynoldsnumber is defined as Re=UDρ/μ where Re is the Reynolds number, U is thegas velocity, D is the diameter of the orifice or the tube, ρ is the gasdensity, and μ is the gas viscosity. So, fluid turbulence can be createdwith a gas flow velocity that is more than a factor to 10 smallercompared to a tube for a comparable diameter. This means good turbulentmixing can be achieved much more readily with a turbulent jet than byturbulent flow in a tube.

In various embodiments of the present disclosure including those shownin FIG. 4 through FIG. 9 and FIG. 13, the cross-sectional shape of thecylindrical chamber is generally circular, but other shapes, such asthat of an ellipse, a square, a rectangle, and other polygonal shapeswith straight interior surface walls, or a combination of shapes formedby straight and curved wall can also be used. The three-dimensionalchamber also does not need to be cylindrical in shape as depicted inFIG. 4. Other chamber shapes such as the shape of a sphere, anellipsoid, a pyramid, a cube, a rectangle with length, width and heightthat are not equal, and other polyhedral shapes formed by flat walls, orwalls with some that are flat and some that are curved. Since thecirculatory gas flow pattern can develop in a wide range of chambershapes, the precise shape of the chamber is relatively unimportant, Therequired circulatory gas flow pattern can develop in many differentchamber shapes and still suffice to provide the needed cooling andmixing to generate a super-saturated vapor in the chamber forcondensation and droplet growth on particles.

The approach described above of using a circulatory gas flow to create asuper-saturated vapor atmosphere in a cold-wall condenser for vaporcondensation and droplet growth is fundamentally different from otherapproaches that have been used in the past for creating vaporsuper-saturation for condensation particle counting. Historically,particle counting by vapor condensation and droplet growth is based onadiabatic gas expansion to cool the gas and create a super-saturatedvapor atmosphere for condensation and droplet growth to occur. The gasflow in such devices is intermittent. The historically importantexpansion type CPC, which has an intermittent gas flow, has largely beenreplaced by the modem continuous flow CPCs similar to that described inU.S. Pat. No. 4,790,650. In contrast to the expansion type CPC whichuses water as a working fluid, the modern continuous flow CPCs areorganic fluid based in which an organic working fluid such as alcohol isused to generate vapor for condensation, with butyl alcohol being themost widely used working fluid. An approach to adiabatic gas expansionfor gas cooling in a continuous flow CPC is described in U.S. Pat. No.6,980,284 in which a vapor saturated gas is expanded through a smalldiameter capillary tube under condition of high pressure drop to createsonic flow and adiabatic gas cooling in the capillary tube.

In recent years, there is an interest in returning to water as a workingfluid because of its abundance and non-contaminating nature. Water isalso considered an “environmentally friendly” substance to use in ameasuring instrument such as the CPC. As pointed out in U.S. Pat. No.6,712,881, water does not work well in the traditional continuous flowCPC, since water would diffuse too quickly to the cold condenser wallsof the laminar flow condenser to cause the gas stream to be depletedwith water vapor by an amount sufficient to prevent super-saturation todevelop for vapor condensation and droplet growth. The laminar flow,cold wall condenser is therefore normally used with an organic workingfluid, such as butyl alcohol, because the relatively lower moleculardiffusivity of the higher molecular weight organic working fluid wouldprevent rapid vapor depletion thereby making it possible for vaporsuper-saturation to develop in the laminar flow stream for condensationand droplet growth.

U.S. Pat. No. 6,712,881 then describes a laminar flow CPC of anon-traditional design for use with water as a working fluid. In thisnon-traditional design, a hot, wet-wall condenser is used in a laminarflow condenser to generate vapor for diffusion into a cold laminar gasstream flowing through the condenser. Water vapor is added to theflowing cold gas during its passage through the hot wet-wall condenserto create super-saturation for vapor condensation and droplet growth.The apparatus has since become a commercially available device. Anotherapproach to using water as a working fluid is the continuous flow devicedescribed in U.S. Pat. No. 5,803,338 in which two water-vapor saturatedgas streams—one hot and one cold—are mixed to create a mixture streamhaving a super-saturated vapor to condense on particles and formdroplets.

The CPC of the present disclosure is similar to the traditional alcoholbased continuous flow CPC in one respect in that hot vapor-laden gascontaining suspended particles is introduced into a cold wall condenserto create vapor super-saturation for condensation and droplet growth.Unlike the traditional laminar flow condenser of the traditional design,the condenser of the present disclosure uses a circulatory gas flow tocool the gas and create vapor super-saturation for condensation anddroplet growth in a chamber under turbulent flow conditions, while thegas flow in the traditional continuous flow condenser is both laminarand uni-directional, thus non-circulatory. In addition, unlike thetraditional laminar flow cold-wall condenser that works well only withan organic working fluid, but not with water, the circulatory gas flowcondenser of the present disclosure works well both with water as aworking fluid, as well as an organic working fluid, such as alcohol.Further, in contrast to the mixing CPC of U.S. Pat. No. 5,803,338, inwhich two water-vapor saturated gas streams—one hot and one cold—aremixed to create super-saturation for vapor condensation and dropletgrowth, the apparatus of the present disclosure uses only one gas streamin a single condenser chamber to create a non-uniform gas stream interms of temperature and vapor partial pressure, which is then mixedthoroughly by fluid turbulence to create vapor super-saturation forcondensation particle counting. The condensing apparatus of the presentdisclosure is therefore simpler, and easier to implement than thetwo-stream apparatus of U.S. Pat. No. 5,803,338. Other mixing type CPCssuch as those described in U.S. Pat. No. 4,449,816 and No. 6,567,157also use a hot and a cold gas stream and mix the two to create theneeded super-saturation for condensation and droplet growth.

The approach to vapor condensation and droplet growth described in thepresent disclosure, therefore, has resulted in an apparatus that isnearly universal in the choice of working fluid for use in theapparatus. Almost any fluid including water and those with a highermolecular weight and a correspondingly lower molecular diffusivity thanwater can both be used. It has also simplified the mixing CPC design byusing a single gas stream in a single condenser chamber to create anon-uniform temperature distribution in a single gas and cause the gasto mix to create a super-saturated vapor atmosphere in the chamber forcondensation and droplet growth.

In the embodiment of FIG. 6, the gas flow entering the condenser chamberthrough the inlet creates a gas jet impinging onto the wall of acylindrical shaped condenser chamber in a normal, i.e. perpendiculardirection. FIG. 8 shows an embodiment with circulatory gas flow byhaving the gas inlet 460 oriented to form a gas stream flowing along theperiphery, i.e. the tangent of the circular section of the verticalcylindrical chamber. This tangential gas flow then moves along path 465to cool the gas, causing it to mix with the incoming hot, vapor ladengas entering the chamber through inlet 460 to create a mixture having asuper-saturated vapor atmosphere to condense vapor on particles and formdroplets. The vortex flow that may develop in such a tangential inletflow device is depicted by arrows, 470, 472, 474, and 476 in FIG. 8. Aspreviously noted in the normal inlet condenser of FIG. 5, the vortexflow will develop only when the volumetric gas flow rate through theapparatus is sufficiently high and the cross-sectional area of the gasinlet is sufficiently small for a gas stream with a sufficiently highmomentum to develop and create the vortex flow pattern in the condenser.At a lower volumetric gas flow rate through an inlet with a relativelylarge cross-sectional area, the full vortex flow may not form, but therelatively slow moving gas flow will still have sufficient momentum tocreate fluid turbulence for mixing and creating vapor super-saturation.The cylindrical chamber in both the one embodiment of FIG. 4 with anormal flow inlet and the other embodiment of FIG. 8 can have anycross-sectional shape, including circular, elliptical, square,rectangular, polygonal, among others. The preferred cross-sectionalshape is circular.

FIG. 9 yet illustrates another example of a circulatory gas flow bylocating gas inlet 420 on the side wall of a vertically oriented,conical chamber 450. Other than the chamber shape, all other aspects ofthe design are similar to the cylindrical chamber design of FIG. 4 withall similar parts being labeled with like reference characters. Theconical shaped chamber can be used advantageously to improve theperformance of the circulatory gas flow condenser and provide a moresmooth aerodynamic flow transition from the larger conical base to thesmaller flow outlet 425 on the top. Other orientations of the conicallyshaped chamber can also be used to achieve specific design advantages.Such advantages will become obvious to those skilled in the art ofdesigning condensation particle counters after having studied thepresent disclosure. Therefore they will not be further described.

The process of using a circulatory gas flow condenser for creating vaporsuper-saturation, condensation and droplet growth can be analyzedtheoretically to aid in the design of the actual apparatus. Although thecomplicated gas flow patterns in a circulatory gas flow condenserprevents an exact analysis to be made, an analysis made under certainsimplifying assumptions can be helpful to improve the understanding ofthe operation of the circulatory gas flow condenser and the parametersaffecting the ability of the device to create vapor super-saturation forcondensation and droplet growth. Although the analysis described belowis made under simplifying assumptions, the result is believed to besufficiently accurate for many practical applications.

FIG. 10 shows graphically the temperature distribution across thestreamlines near the end of the flow path 450 in FIG. 6 along thedirection of line 448, which is perpendicular to the cylindrical wallsurface at that point. As one moves from point A to point B along line448, one would move from the relatively cooler region close to the wallto the relatively warmer region further away from the wall. As depicted,the gas temperature at point A would be the same as the temperature ofthe wall, T₂. At the outer edges of the streamlines at point B and undersuitable operating conditions one encounters circulating gas flow alongthis outer boundary of the gas flow away from the wall. Gas flowingalong this outer boundary is not in contact with any cold wall surface.Therefore, it can lose heat only by turbulent mixing or thermaldiffusion to the cooler stream flowing closer to the cold condenserwall. Again under suitable operating conditions, the gas stream alongthis outer boundary would lose very little heat or vapor to the coldcondenser wall. As a result, the temperature at point B is not greatlydifferent or is substantially the same as the temperature of the gas,T₁, entering the chamber through inlet 420. This is depicted by curve 1in FIG. 10.

The line connecting the ends of Curve 1 is a straight line, Line 2, onwhich one can depict the bulk temperature, T_(b), of the gas, followingthe turbulent mixing of the non-uniformly cooled gas stream in theturbulent vortex core, a region of space occupied by the sequences ofarrow 452, 454, and 456, and the center, 460 of the vortex core in FIG.6. The bulk temperature of a gas with a non-uniform temperaturedistribution is the temperature of the gas following thorough mixing ofthe gas under adiabatic conditions. Mixing is adiabatic if no heat isadded or removed from the gas during the mixing process. Point C on Line2 shows the bulk temperature, T_(b), of the gas under one specific setof operating conditions of the apparatus.

As the gas flows along path 450 in FIG. 6, the stream line moving closeto the cold wall surface will quickly cool to the temperature of thewall, T₂. Vapor will also condense from the gas stream, so that thevapor pressure of the gas will be the saturation vapor pressure of waterat T₂. Gas flowing along streams at varying distances from the coldcondenser wall will lose varying amounts of vapor by vapor diffusion asthe individual streams reach the end of the path 450. The vapor pressureof water in the overall gas stream will thus also be non-uniform. In theturbulent vortex core, the main stream will be thoroughly mixed,creating a uniformly mixed stream having a bulk partial pressure ofwater, p_(b). the bulk vapor partial pressure being the partial pressureof vapor in the bulk gas after thorough mixing without gaining or losingvapor during the process.

The rate of heat loss by a gas flowing adjacent to a cold wall isdependent on the thermal diffusivity of the gas and the nature of thegas flow. The rate of vapor loss depends on the molecular diffusion ofthe vapor in the gas and the nature of gas flow. The major resistance toheat and vapor loss to the cold condenser occurs in the laminar boundarysub-layer near the wall. In the laminar sub-layer, the rate of heat lossand vapor loss are proportional to the thermal diffusivity of the gasand the molecular diffusivity of vapor in the gas. For water vapordiffusion in air, the vapor diffusivity is not greatly different fromthe thermal diffusivity of the air. For simplicity, they are assumed tobe equal. As a result, the same straight line, Line 2, connecting theends of Curve 1, also depicts the bulk vapor pressure followingturbulent mixing in the vortex core.

Curve 3 of FIG. 10 depicts the saturation vapor pressure of water at thebulk temperature, T_(b), of the gas. Since vapor pressure increases morerapidly with increasing temperature, Curve 3 is a concave curve facingupward as depicted. This shows that the bulk vapor pressure, p_(b), inthe gas following mixing is higher than the saturation pressure, P_(s),at the bulk temperature of the gas/vapor mixture. The ratio of p_(b) andp_(s) is the saturation ratio

$S = \frac{pb}{ps}$

When S is less than 1.0, the gas is said to be under-saturated, andwhen, the gas S=1.0 is saturated. When S is greater than 1.0, the gas issaid to be super-saturated. As shown in FIG. 10, the gas created bymixing a stream having a non-uniform temperature and vapor concentrationis super-saturated.

FIG. 11 shows the result of a theoretical analysis based on the aboveapproach. The theoretically calculated saturation ratio, S, is plottedagainst the cooling parameter,

$f = \frac{{T\; 1} - {Tb}}{{T\; 1} - {T\; 2}}$

where T₁ is the temperature of the saturated gas entering the condenser,T₂ is the temperature of the condenser wall, and T_(b) is the bulktemperature of the cooled gas produced by the circulatory gas flow alongthe cold wall condenser.

The result of the above analysis shows that if the volumetric rate ofgas flow through the condenser is too high, and the residence time ofthe gas in the circulatory gas flow condenser is too short, there isinsufficient time for the gas flowing through the condenser to be cooledto a significantly lower temperature. In which case, T_(b)≈T₁, f≈0, andS≈1.0 as shown in FIG. 11. Similarly, if the volumetric rate of gas flowthrough the condenser is too small, and the residence time of the warmvapor-saturated gas flowing into the chamber is too long, the gas wouldhave cooled significantly and substantially to the same temperature asthe condenser wall, i.e. T₂. In which case, T_(b)≈T₂, f≈1.0, S≈1.0.Again, so super-saturation can develop, as shown in FIG. 11.

The result of FIG. 11 shows that super-saturation can develop only whenthere is partial cooling of the gas to an intermediate temperature, sothat T₂<T_(b)<T_(l), or 0<f<1.0, and that the maximum super-saturationis developed when f≈0.7 for the specific example shown. Thus to developmaximum super-saturation for condensing vapor on the smallest particlesize possible, the condenser chamber should be designed to achieveapproximately 70% of the maximum cooling possible.

The relationship between the saturation ratio, S, and the minimumdiameter, d, of particles on which vapor will condense to form dropletsis governed by the Kelvin equation,

$S = {\frac{P}{Ps} = {\exp \left( \frac{4\sigma \; M}{\rho \; {RTd}} \right)}}$

where σ is the surface tension of the liquid working fluid, M is itsmolecular weight, ρ is its density, R is the gas constant of the gas, Tis the temperature, and d is commonly referred to as the Kelvinequivalent diameter. FIG. 12 shows the relationship between thesaturation ratio and the Kelvin equivalent diameter for water. Theresult shows that the saturation ratio needed to cause vaporcondensation on 10nm diameter particles is S=1.27. For condensation on5.0 nm particles, a saturation ratio of S=1.62 is needed and to condensevapor on 2.0 nm particles, a saturation ratio of S=3.31 would benecessary.

Like most analyses, the analysis made in this disclosure is not exact,and can be improved using more sophisticated methods. However, theability of the circulatory gas flow condenser to create vaporsuper-saturation for condensation on particles does not depend on theaccuracy or even the validity of the method used in the analysis.Laboratory experiments have shown that vapor super-saturation is indeedachieved and that the device of this invention is effective in creatingvapor super-saturation for condensation particle counting using water asa working fluid, as well as an organic working fluid with highermolecular weight and correspondingly lower molecular vapor diffusivitythan water.

FIG. 13 shows yet another approach to creating vapor super-saturationfor condensation on particles to form droplets for detection and/orcounting. FIG. 13 is similar to FIG. 5 except gas cooling isaccomplished by a laminar gas stream flowing in a cold tube. In thisembodiment, warm gas containing vapor and particles is introduced into acooler tubular shaped chamber 410 with a generally circularcross-section. Upon entry into chamber 410 through inlet 480, the gasflows upward in the chamber as indicated by arrows 485. This flowing gasthen loses heat by thermal diffusion to the cold chamber wall. At thesame time vapor is lost to the wall by molecular diffusion andcondensation on the wall. Streamlines that are close to wall 440 willlose heat and vapor more quickly than those that are farther away. As aresult, the temperature of the gas and the partial pressure of vapor inthe gas will become non-uniform as the gas flows upward in the chamber

Upon reaching the end of the chamber, this non-uniformly cooled gas witha non-uniformly distributed vapor partial pressure in the gas then flowsout of chamber 410 through flow restriction 425 to form a turbulent gasjet in chamber 426. The turbulence causes the gas to mix, thus makingboth the gas temperature and the vapor partial pressure in the gas moreuniform, resulting in the creation of a super-saturated vapor whichcondenses on particles and forms droplets. The process is similar tothat in the circulatory gas condenser of FIG. 4 except in the preferredembodiment of FIG. 4, cooling and mixing of the gas takes place in asingle chamber, whereas in the embodiment of FIG. 13 two separatechambers are used, one for gas cooling and the other for mixing.

The embodiment of FIG. 13 is equivalent to that of adding a mixingchamber downstream of a conventional laminar flow cold-wall condenser.While the traditional laminar cold wall condenser by itself cannotcreate vapor super-saturation to form droplets when water is used as theworking fluid as discussed in U.S. Pat. No. 6,712,881, the embodiment ofFIG. 13 has made it it possible to create a super-saturated vapor tocondense on particles and form droplets for counting when water is usedas the working fluid.

We have described in this disclosure various approaches to creatingvapor super-saturation for condensation particle counting using a widerange of working fluids including water and chemical substances with ahigher molecular weight and a lower vapor diffusivity than water. Thisdisclosure will enable individuals skilled in the art of condensationparticle counter design to use the principle described herein to createCPC designs beyond those described in the present disclosure. Thesedesigns, therefore, will not be further described or discussed.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. An apparatus for condensing vapor on particles to form droplets fordetection, said apparatus comprising: a chamber with an inlet, thechamber being capable of being held at a first temperature and foraccepting gas passing through the inlet, the gas being at a secondtemperature, different from the first temperature, the gas entering thechamber such that the difference between the first temperature and thesecond temperature will cause the gas upon mixing to produce asuper-saturated vapor atmosphere for condensation on said particles toform droplets, said chamber including an outlet for said gas to flowthrough and exit thereby carrying said droplets so formed in saidchamber for detection by a droplet detector located downstream.
 2. Theapparatus of claim 11, including a flow restriction dividing saidchamber into an upstream compartment and a downstream compartment, suchthat the gas when entering said inlet will enter said upstreamcompartment, then flow through said flow restriction to create aturbulent gas jet in said downstream compartment to cause the gas tomix.
 3. The apparatus of claim 11, wherein said chamber is configured toaid in the development of a circulatory gas flow in said chamber tocause said gas temperature to change and said gas to mix.
 4. Theapparatus of claim 11, including an additional chamber comprises aporous wall for liquid to fill interstitial pore spaces of said porouswall thereby generating vapor for saturating said gas flowing along saidporous wall.
 5. The apparatus of claim 11, wherein said chamber beingsubstantially cylindrical in shape.
 6. The apparatus of claim 15,wherein said substantially cylindrically shaped chamber has asubstantially circular cross-section.
 7. The apparatus of claim 11,wherein said inlet being orientated to create a gas jet impinging onsaid wall of said chamber in a substantially perpendicular direction. 8.The apparatus of claim 11, wherein said inlet has an orientation tocause said gas to flow in a substantially tangential path along saidwall of said substantially cylindrically shaped chamber.
 9. Theapparatus of claim 11, wherein said chamber being substantially conicalin shape.
 10. The apparatus of claim 11 including a mechanism to controlthe second temperature, said mechanism including a thermoelectric modulein thermal contact with said wall of said chamber.
 11. The apparatus ofclaim 20 including a temperature sensor for controlling said secondtemperature to a desired set-point value.
 12. An apparatus forsaturating a gas containing particles with vapor for said vapor tocondense on said particles to form droplets for detection, saidapparatus including a chamber for being partially filled with a liquidto form a liquid surface in said chamber, said chamber including aninlet for said gas to enter and fill the space above said liquidsurface, and a plurality of porous panels partially immersed in saidliquid to accept liquid in the porous panels, thereby permitting saidliquid to evaporate from said wetted porous panels for forming vapor insaid gas above the liquid surfaces, said chamber including an outlet forsaid gas carrying said vapor to exit said chamber.
 13. The apparatus ofclaim 22, wherein said porous panels are spaced apart to provide morethan one passageway for said gas to flow through, with said wettedporous panels forming vertical side walls of said passageways toevaporate liquid and form vapor to saturate said gas flowing throughsaid passageways.
 14. The apparatus of claim 22, wherein said chamberhas a mechanism for keeping a top portion of the chamber wanner than alower portion of the chamber.
 15. The apparatus of claim 22 and furthercomprising a droplet detector that comprises an optical, lightscattering detector.
 16. The apparatus of claim 25 wherein the dropletdetector comprises an optical, light scattering detector.
 17. Theapparatus of claim 22 including an adjustable-speed pump to adjust arate of flow of said gas through said inlet into said chamber to aselected value.